There has conventionally been a monitoring camera that has two types of modes, i.e., a day mode in which imaging is performed under illumination of visible light such as natural light in the daytime or white light and a night mode in which imaging is performed while irradiation with infrared light is being performed.
In imaging in the day mode, it is possible to obtain a color image. Meanwhile, in imaging in the night mode, reflected light obtained by causing infrared light with which irradiation is performed to be reflected by a subject is received, and therefore it is impossible to obtain color information such as R, B, and G and it is possible to acquire only received light intensity of the reflected light (infrared light). Therefore, a resultant image is a monochrome image of gray, green, or the like.
By the way, in a case where use of the monitoring camera is considered, it is desirable to obtain a color image even in a case of imaging in the night mode.
In view of this, there have conventionally been proposed various methods of generating a color image on the basis of an image obtained by imaging a state in which an imaging range (subject) is irradiated with infrared light (hereinafter, referred to as “infrared image”).
For example, Patent Literature 1 discloses a method of generating a color image on the basis of an infrared image obtained by performing irradiation with three types of infrared light having different wavelengths. This method uses a fact that reflectivity obtained in a case where resin is irradiated with infrared light having wavelengths of 780 nm, 870 nm, and 940 nm has a positive correlation with reflectivity obtained in a case where irradiation with visible light of R, G, and B is performed. According to this method, it is possible to obtain a color image having high color reproducibility in a case where a subject is resin.
However, in a case where a subject is not resin, an R component of a color image to be generated has comparatively high reproducibility, but, regarding G and B components, original color cannot be reproduced. Therefore, this method cannot be applied to an arbitrary subject other than resin.
Further, for example, Patent Literature 2 proposes a method of performing irradiation with infrared light and performing imaging in a state in which a subject is slight 1, y visible under, for example, illumination of low-illuminance light. In this method, an image in which visible light and infrared light are mixed is imaged, and color information is reproduced by changing a parameter of white balance processing applied to this image from a value applied to a normal image of only visible light.
However, this method has an essential problem that infrared light and visible light cannot be separated. Therefore, accuracy of color information to be reproduced is reduced due to an influence of infrared light.
Furthermore, for example, Patent Literature 3 and Non-Patent Literature 1 disclose a technology of continuously imaging an infrared image obtained in a state in which irradiation with infrared light is performed and a visible image obtained under low illuminance, i.e., in a state in which irradiation with infrared light is not performed and generating a color image by using those images.